This invention relates generally to top/bottom pincushion correction circuits for raster displays on cathode ray tubes (CRTs) and particularly to active top/bottom pincushion correction circuits. The above-mentioned copending applications relate to active pincushion correction circuits that are especially adapted for use in color monitors used as computer output display devices. Specifically, the CRT display devices are constructed with substantially flat faceplates. Application Ser. No. 000,694 is directed to independent top/bottom pincushion correction circuitry in which the pincushion correction signal for the top portion of the raster may be separately tailored from the pincushion correction signal for the bottom portion of the raster to meet the exacting requirements of modern day monitors. Application Ser. No. 000,695 relates to side pincushion correction circuitry that tracks with changes in raster size. The present invention may advantageously be used with either or both of the inventions in the copending applications. Its benefits, however, are independent and the invention may be used alone, if desired.
In both of the copending applications, multipliers are utilized to develop the output pincushion correction signals. The multipliers include an array of differentially coupled transistors and current sources and have modulating inputs and carrier inputs and produce differential outputs consisting of the multiplied input signals. In Ser. No. 000,694, a gain control input on the multiplier is coupled to a voltage that is representative of the horizontal size of the raster and the vertical ramp signal is applied to the modulating and carrier inputs of the multiplier. Since the vertical ramp signal is dependent upon the vertical size of the raster, the pincushion correction signal tracks with vertical raster size changes. In the present invention, the top/bottom pincushion correction multiplier has its gain control terminal coupled to receive a voltage that is indicative of horizontal raster size changes and thus enables automatic tracking of changes in raster size for the top/bottom pincushion correction signal.